Certain analyzer incubators, such as those shown in U.S. Pat. No. 4,298,571, comprise a rotor that is an annular disk that has a lower support surface for test elements and an evaporation cover at each station around the circumference of the disk. (The specific incubator shown in the '571 patent is for potentiometric test elements, but a similar one has been used for colorimetric test elements in commercial instruments.) The entire disk is cast as a single piece, necessitating castings of the finest quality, in order to ensure that each support surface (for each test element) is flat and at a predictable height in the "Z" dimension. That is, little or no warpage is tolerated. The reason for the strict control of height tolerances is that for some incubators, the test elements held on such support surfaces, are read in place at the respective incubator station by a built-in reflectometer, particularly for those incubators requiring multiple readings of the same test elements (the so-called "rate" test elements). Any variation in height (the Z dimension) of the support surface away from its predictable location, called a "processing" error due to the disc wobbling as it rotates, alters the amount of light falling onto the test element from the reflectometer. This in turn creates an error in the colorimetric reading.
It is known that rate test elements are read at the first derivative level, so that errors introduced into the absolute readings disappear. However, some test elements are colorimetrically read only at their end point. If the same incubator is to be used for both rate test elements and such end-point elements, a desired goal to simplify analyzers, an improvement is needed in the rotor to avoid the noted precessing error.
Not only does such needed accuracy in reading cause manufacturing problems in making such rotors, but in addition, the rotors are very expensive. This leads to a substantial loss if such rotors are defective or break for any reason.
Yet another drawback with the rotors of the '571 patent is that the test element holding means is not readily removable from the rotor, due to the manner in which the hold-down spring is attached.
There has been a need, therefore, prior to this invention, to provide an incubator in which the rotor provides support surfaces that more readily insure a predictable height of test elements from the surface on which the rotor rotates, without requiring careful production of the entire rotor; or a more readily removable test element holding means.
One solution to this problem has been to provide a rotor that has no supporting surface underneath the test elements, which instead pushes test elements around directly on a stationary incubator surface. Examples are shown in, e.g., Japanese Kokai 86/259142. For low-throughput volumes, such is not a problem. However, high-throughput analyzers rotate the rotors very rapidly, with considerable acceleration. If, as is common, the test element is manufactured from a plastic material such as polystyrene that does not wear well, there is considerable polystyrene dust created that jams up the incubator, requiring repeated cleaning.
Thus, the need for an improved rotor has remained unmet, at least regarding high-throughput analyzers. (As used herein, high-throughput means at least 200 test elements per hour.)